ExistingSiteBase::prepareRequest is not compatible with UiHelperTrait::prepareRequest
When extending from ExistingTestBase, and also using UiHelperTrait (to alias a function) an error is thrown since ExistingTestBase::prepareRequest has added a return type that does not exist on UiHelperTrait.
Example code:
abstract class MyTestBase extends ExistingSiteBase {
use UiHelperTrait {
drupalGet as traitDrupalGet;
}